Can prescribed fires restore C<sub>4</sub>grasslands invaded by a C<sub>3</sub>woody species and a co‐dominant C<sub>3</sub>grass species?
R. James Ansley, Thomas W. Boutton, Emily B. Hollister
Abstract
Abstract Prescribed fire is used to reduce woody plant invasion and restore herbaceous production and diversity in grasslands and savannas worldwide. Here, we determined whether a concentrated series of repeated‐winter, repeated‐summer, or alternate‐season (winter and summer) fires in a short timeframe (“transition fires”) could catalyze the restoration of C 4 perennial grasses in Southern Great Plains, USA grasslands that had become dominated by a fire‐tolerant C 3 woody N 2 ‐fixer (honey mesquite, Prosopis glandulosa ) and a C 3 perennial bunchgrass (Texas wintergrass, Nassella leucotricha ). We applied transition fires over a 5‐yr span and maintenance fires on a portion of each plot 7 or 8 yr later. We measured herbaceous standing biomass and cover and soil variables (soil organic C, N, δ 13 C, and δ 15 N) in unburned, transition‐burned, and maintenance‐burned treatments. Greater δ 13 C at 10–20 (−17‰) than 0–10 (−20‰) cm depth increment confirmed that vegetation was historically mostly C 4 grassland that shifted toward C 3 dominance. Transition treatments with summer fire were most effective at top‐killing mesquite, but no treatments root‐killed >3%. Regrowth of top‐killed mesquite was similar in all treatments and reached pre‐fire height by 9–10 yr post‐fire. Herbaceous production and cover responses showed that: (1) Alternate‐season transition fires increased C 4 mid‐grass, but did not change Texas wintergrass; (2) repeated‐summer fires reduced Texas wintergrass, but did not change C 4 mid‐grass; and (3) repeated‐winter fires did not change C 4 mid‐grass or Texas wintergrass compared with the unburned control. All maintenance fires stimulated Texas wintergrass biomass and cover, thus eliminating the reduction of Texas wintergrass caused by repeated‐summer transition fires. There were no long‐term effects of transition fires on soil C, N, δ 13 C, or δ 15 N. Results advance our understanding of the expectations and limitations of prescribed fire in shifting a woodland alternate state toward what was historically a fire supported C 4 grassland/savanna.